Receivers in a global navigation satellite system (GNSS), such as the Global Positioning System (GPS), use range measurements that are based on line-of-sight navigation signals broadcast from satellites. A receiver measures a time-of-arrival of one or more broadcast signals. This time-of-arrival measurement includes a time measurement based upon a coarse acquisition (C/A) coded portion of a signal, called pseudo-range, and a phase measurement.
The presence of interference signals may degrade a signal-to-noise ratio (SNR) of one or more GNSS navigation signals. FIG. 2 illustrates continuous wave (CW) interference 200. The CW interference 200 can be viewed as an interfering signal, in this case sinusoidal, superimposed on a spread-spectrum signal 210 used by a respective GNSS signal. Before the spread-spectrum signal 210 is correlated in a receiver, an amplitude of the interfering signal is often significantly larger than an amplitude of the spread-spectrum signal 210. Correlation despreads an energy of the GNSS signal and spreads an energy of the interfering signal, which then becomes noise-like. If additional noise from the despread interfering signal is larger than an ambient background thermal noise 212, the SNR of the received GNSS signal is decreased.
The SNR of the GNSS signal varies with a local amplitude of the interfering signal. In addition, the background thermal noise 212 mask the spread-spectrum signal 210. When quantized, however, the spread-spectrum signal 210 is more easily detected at crests and troughs of the interfering signal, where a rate of change of the interfering signal is nearly zero. The spread-spectrum signal 210 is more difficult to discern at a maximum of a magnitude of the rate of change of the interfering signal.
One conventional anti-jamming approach used 3-level quantization 214 based on the amplitude of the interfering signal such that samples near the crests and troughs of the interfering signal are used during signal processing in the receiver. 10 to 20% of the samples that fall at the crests are weighted +1 and 10 to 20% that fall at the troughs are weighted −1. Remaining samples are discarded by giving them a weight of 0.
It may, however, be difficult to achieve desired sample populations in this conventional anti-jamming detection approach. There is a need, therefore, for an improved anti-jamming detection scheme in GNSS receivers.